Rectifier-powered equipment



April 28, 1953 L J. HIBBARD RECTIFIER-POWERED EQUIPMENT Communcoion f8 System Iam , telaio y22ML *i I Y 'Y Fig.|. Im

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WITNEssEs; Fig' 2- lNveNToR d 727 M7 oyd J.Hilbbord. 2%/- W 9W ATTORN EY April 28, 1953 L. J. IIBBARD 2,636,997

RECTIFIER-POWERED EQUIPMENT Filed Feb. 10, 1951 2 Sl-iEETS-SHEET 2 A 8 Communication System 56 36 3 3| FILL@ ll-MJ [5w l 32 37 36" 2: 22 --II-'m-4 33 4 35 i 35 t f L V34 V3I Fig.5. wlTNEssEs: INVENTOR 54.72%/ Lloyd J.Hi,bbord.

BY 720, mw

ATTORNEY Patented Apr. 28, 1953 RECTIFIER-POWERED EQUIPMENTv Lloyd J. Hibbard, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 10, 195-1, Serial No. 210,390

Claims.

My invention relates to rectifier-powered trolley-energized equipments, particularly rectierpowered traction-motor equipments `for electricrailway and'othervehicles.- My invention relates to Vthe filtering means and the series-reactance means, and to combinations of both, whereby the performance of the vehicle-driving motors is improved, whereby-thewave sh-ape of the alternat-y ing-current supply-line `current-is improved by the-removal or reduction of low-order harmonics,

and `whereby I also 4remove or-reduce the higherorder harmonicswhich cause inductive interference with communication-circuits which parallel the alternating-current supply-line.

My present invention is an improvement `over the broad Vsubject matter-which is described and claimed inl my application Serial No. 120,331, filed October 8, 1949,- for Inductive-Interference VPreventive-Means, in which-a direct-current loadcircuit, containing the'traction-motor, and having a large direct-currentripple, is energized, from a single-phasetrolleyA which is subject` to inductive interferenceconsi'derations, through a rectiiier-system under Ysuch conditions -thatthere is enough series inductance,"on the supply-circuit side of the rectiers, to' produce aA substantial angle of overlapbf the Yhalf-wave rectiiier-currents, during each `half-cycle "of the-'supply-current. Where'it is `necessary (as is usually the case) a harmonic-reducing iilter isalso provided, as described and claimed lin lmycopending application just mentioned.

My present'invention'also involves improvements over 'my copending application Serial No. 140,475, led January 25,'1950, for Dephased Inductive-InterferencePrevention, in which the a1- ternating-current dsupply-currents VAwhich are fed into a plurality lof rectifier-powered tractioninotor assembliesare caused `to be slightly outof phase with each other,"soA that certain high-irequency harmonics will be approxinrately'180o out of phase with eachother; orv so'rel'ated'as to vconstitute a practically' balanced"polyphasesystem, which reduces Ythe line-harmonicsto a low value in the frequency-range which is the most 'critical irom the standpoint of inductive4 interference.

-As most trolley-energized motor-driven vehicles are provided with a'plurality-ofl driving or traction motors, my present invention involves the provision of a plurality of Arectiii'er-assemblies on.

each vehicle, each powering its own motor or group of motors, ,and4 each involving its own anode-reactance which more eiectively snuiis out backfire-currents, and provides lmore orbetter series-reactance for voltage-regulation during lll low-voltage motor-operation, and provides a ready means for dephasing .theiobjectionable interference-causing` supply-line harmonics according to the .principles covered in my last-mentioned copending application.

My present .invention also relates broadly tothe use of va specialvanode-reactor for each rectiiierassembly, saidanode-reactor being in theiorm of a reactor-transformer havingtwo closely coupled, substantially identical-turn windings, on a com- .mon magnetic circuit which includes van air gap andwhich preferably also includes someiron or other magnetizable material. The two reactorwindings are serially included in the respective anode-leads ofthe two rectiers with Awhich they areassociated, and are connectedin such polarity that,the.tworreactor-windings are in series for the` commutating-currents which flows between the two` rectifiers during their-'overlapping conducting periods, and also in series for the backfiring` currentsv which iiow4 during .backfire-conditions ThisY series reactor-winding relationship thus produces a maXimumrea-ctance in the reactor-transformer, which is desirable in order` to obtainafhighangle of-overlap during the coinmutating-conditions and to reduce the magnitude of vthe backfire-currents, while reducing the .amount of voltage-reducing reactance which is effective during `the normal load-carrying-periods,

YandatrtheV same time. reducing the overall size ofthe two .reactor-windings.

When Ythere'area plurality of rectiier-powered loads, energized from the same alternating-current 1eadsthe presence. of individual anodevreactors, individual to each rectier-assembly, v

claimed in my first-mentionedcopending application; .These filters are what I call parallel iiiters, being untunediilters consisting of a capacitor in series with a damping resistance, the .capacitor beingtuned to-parallel resonance with the supply-circuit inductance, as seen from the terminals ofthefllter, at a leduency suchas to reduce an-interference-provoking range 0f mg@ .order harmonics.

According to one aspect of my present invention, the parallel filters of the different rectiiierassemblies on any locomotive or car, or on any group of parallel-energized cabs or cars, can be differently connected, with respect t the anodereactance, such as being connected with some filters on the supply-line side of the anodereactance, and some on the rectifier-side, or some crisscrossed with one terminal on the powersupply side of one anode-reactor winding, and the other terminal on the rectifier-side of the other anode-reactor winding. In this way, particularly if the reactance of the step-down transformer is relatively low, it is possible not only to filter out the high-frequency interferenceprovoking supply-current harmonics drawn by each rectifier-assembly, and to throw said highfrequency harmonics out of Vphase with each other, in a certain critical range or ranges of interference-provoking harmonics, but it is also possible to throw the low-order harmonics out of phase with each other, so as to reduce the amount of third harmonic, and other low-order harmonics, which are drawn from the line by the plurality of rectifier-assemblies which are mounted on each vehicle, thus improving the Wave-form of the power-line currents.

One of the disadvantages of the parallelresonant lter is that it aggravates the low-order harmonics, which have frequencies lower than the parallel-resonant frequency of the filter, and which do not cause much telephone-interference, as a price which had to be paid for reducing the high-order harmonics which caused the really serious telephone-interference in the communication-lines which parallel the right of way of the trolley-wire and its alternating-current feeders. The diverse connections of the parallel filters, on different sides of the several anode-reactors, constitute an effective means whereby these low-order harmonics can be thrown out of phase with each other, so that they more or less cancel each other, thus considerably improving the wave form of the supplycurrents, particularly when the different rectifier-powered motors are energized from different step-down transformers.

A further feature of my present invention involves the use of one or more damped series filters, in addition to the above-described parallel filter. Each series filter includes a capacitor and an inductor which are tuned to be in series resonance at a frequency which is either of the approximate order of range of the associated parallel filter, or at some lower frequency-range.

The two or more filters, all connected across the anode-leads or power-supply leads of the two rectifiers of any given rectifier-assembly, have individual filtering effects, in addition to any composite or joint filtering effects which they may have. In other words, the ltering effect of the plurality of filters is not merely the joint effect which would be obtained by mathematically calculating the impedances of the several filter circuits connected in parallel with each other and producing a joint effect which is different from the filtering-effect of any one of the filters acting alone. On the contrary, each filter produces its own effect, somewhat as if the other filter or filters were not present. I am not sure whether this is due to the presence of damping resistors in each filter, or whether it is due to the rectifier-circuit application in which the filters are more or less short-circuited during each commutating period of the rectifiers, during each half-cycle of the supply-line frequency. At any rate, the individual ltering effects are present, and are very useful.

As has been stated, the parallel filters, which are needed to reduce the high-order harmonics which cause the objectionable inductive interference, accomplish this result only at the expense of aggravating harmonics in the vicinity of the parallel-resonance of said filters, and also aggravating all harmonics of a still lower order. By my present invention, it is possible, by the use of series filters, to remove these aggravated harmonics, or as many of them as may be necessary.

With the foregoing and other objects in view, my invention consists in the circuits, systems, combinations, apparatus and parts of apparatus, as hereinafter described and claimed, and illustrated in the accompanying drawings, wherein:

Figure 1 is a much simplified circuit-diagram, indicating circuits and apparatus which illustrate the general principles of some of the aspects of my present invention, particularly the `supplementary effects of the parallel and series filters;

Fig. 2 is a curve-diagram which crudely illustrates the contrasting impedance-effects of my parallel and series filters, at the different frequencies corresponding to the different harmonics;

Fig. 3 is a simplified diagrammatic View illustrating the importance of the anode-reactor in a combination in which motor-control is obtained by means of taps on a power-supply transformer which is interposed between the trolley and the rectifier-equipment;

Fig. 4 is another simplified circuit-diagram, illustrating the eifects of diverse filter-connections with respect to the anode-reactors, and

Fig. 5 is a curve-diagram showing the contrasting oscillographic records of supply-current wave-shapes which are obtained with dversely connected parallel filters.

In the much simplified diagrammatic circuit of Fig. l, I have shown, by way of example, a single-phase power-supply circuit, one side of which is connected to, and includes, a trolley wire l, while the other side of the supply-circuit is grounded, as indicated at 'I'. The trolley wire 'I is paralleled by a communication circuit 8, such as a telephone circuit, which makes it essential to consider the inductive interference effects of the power-line harmonics in the telephone circuit, The power-supply frequency may be either a previously known railway-electrification frequency, such as '25 cycles, or it may be a usual commercial frequency, such as 60 cycles, such as is used for commercial power and light circuits.

In Fig. 1, I have diagrammatically indicated some of the essential electrical parts of a single locomotive or railway-car, which is energized from the trolley Wire 7 through a pantograph 9 and a step-down power-supply transformer Il for that particular locomotive or car. The power-transformer l l is provided with a secondary winding l2 which is used to energize three traction-motor equipments, represented by three direct-current motors I, 2 and 3. Each motor comprises an armature i3, a main series field winding M, an interpole winding i5, a reversing switch I6, and a serially connected reactance or choke coil l1. The negative terminals of each of these motor-equipments are connected to a common negative conductor I8 of the direct-current load-circuit, this negative conductor i8 being connected to the midpoint I9 of the powertransformer secondary l2.

The positive terminal of each of the motor S assemblies i, 2 and 3 is supplied with power from its. own rectier assembly. Thus, each positive motor-terminal is connected to the common cathode-terminal 20(0f two diagrammatically indicated rectiers 2| and 22, each of which may b e an ignitron or other single-phase rectitying device of a type which becomes substantially nonconducting, after a conducting period, `only in response to a current-decrease to substantially zero. In other words, once a current-conducting arc is formed in either one of therectier-devices `2| yor 22, that arc will sustain itseli, and will not become extinguished, until substantially currentn zero. The rectiers 2| and 22 have anode-leads 2| and 22', respectively, which are connected to the opposite-potential terminals of the secondary winding l2 oi the power-transiormer E i. Preierably,vin accordance with my present invention, these anode-leads 2| and 22' include serially connected anode-reactor windings 2i and 22, respectively.

Preferably also, in accordance with my present invention these anode-reactor windings 2l and 22 are-mutually coupled, on a common magnetic circuit, which :may be all air, but which preferably comprises both magnetizable material and an air gap. The two reactor-windings ill and 22" are preferably coupled as closely as is practicable, as by having ktheir respective coils interleaved with each other. There are the same Anumber of turns ineach of the reactor-windings.

The polarity of the reactor-winding connections is such that the two reactor-windings 2li and 22 are Vin series-circuit relation to each other, producing a vmaximum reactance in the reactortransformer of which they are a part, for the commutating currents which flow between the two rectifying devices 2l and 22 during their overlapping conducting periods.

The overlapping conducting periods of the two rectifiers 2| and 22 are caused by the supply-line inductance of the supply-circuit up to the rectifiers. This supply-circuit inductance includes the internal reactance of the alternating-current generator and any transmission line and powerline transformers associated therewith (not shown), the reactance of the trolley conductor 'l up to the load, the reactance of the powertransformer H on the vehicle, and the reactance of the reactor-transformer halves 2l and in series, which are both carrying current during the `overlap-period. Preferably, in practicing my invention, the power-transformer i i has as small a-leakage-reactance as practicable, so that it merely serves as a voltage-reducing means, and usually also has a voltage-varying means, as shown in Fig. 3, wherein the power-transformer il is provided with secondary voltage-changing taps 23 and 24. Preferably also, in practicing my invention the amount of supply-circuit reactance which is supplied by the two serially conheated-halves 2i" and 22 of the anode-reactor transformer, is about of the same order of magnitude as the rest-of the supply-circuit inductance, .up to these anode-reactors, although I am not limited to this detail.

It is frequently -a desirable feature of my present invention to make the plurality of anodereactor transformers 2lf-22', which are .associated with the respective rectifier-assemblies of the locomotive-equipment, have different reactances, such'as by having different numbers of turns, as has been diagrammatically indicated, in Fig. 1, by means of reactor-taps25 dierent `taps being brought into play for .the three different motor-equipments which are illustrated in Fig. 1.

The eiect of these different taps -sto cause .the power factors of the alternating currents drawn by the three different motor-loads to be slightly different from each other, thus causing a small phase-difference between the single-phase supply-currents of the respective-motor-loads. This small phase-difference, when multipliedlby the harmonic order-number of Vthe harmonic 'which causes the most trouble, fromlthe standpoint .of

inductive interference, will (or may) produce a harmonic-current'phase-difference which is large enough -to substantially cancel out Athat particular harmonic. yand to materiallyreducethe magnitudes of the harmonics in the adjacent frequency-range, as set forth inmy second-mentioned copending application, whichV has already been identiecl.

While I have described. my rectifier-assemblies, for convenience in description, asif their cathodes were connectedto one motor-terminal 2U, and as if their anodes were connected to, Vor energized by, kthe power-.supply leads which I have designated asthe anode-leads .21' and22f, in accordance with acommon rectifier-connection, it will be understood,` of course, that these rectifier-polarities might vbe reversed. Furthermore, while I have described. and illustrated rectifier-assemblies, each `of .whichcomprises only two rectiers 2l and 22, which areusedto energize onlyone of the terminals.. of the directcur-rent load-circuit, the other .direct-current terminal beingbrought vrback to the :midpoint I9 of the supply-transformer secondary i2, it will be understood that the return-circuit I8 for the direct-current loadcircuit could be brought back to the power-supply means inanyother known way, such as by meansof .the secondhalf of va rectifier-bridge connection (not shown).

In Fig. l, I also show theluse ofboth parallel and series lters, in accordance vwith my present invention. These filters are connected across the respective pairs of .anode-leads 2l-22 of the respective rectifier-assemblies .which are used in any particular locomotive orother trolleypowered vehicle using :my invention. In Fig. l, these anode-lead lter-connections..are shown as being on the rectifier-sides of the anodereactor transformers `2l-22" of `the respective rectifier-assemblies, although, as will be discussed more explicitly in connection with Fig. 4, the nlters could be connected toany otherportions ofthe anode-leads 2li-.22.

In each or' the three rechner-assemblies which are shown in Fig. l, the .rstior top) ltercircuit is a parallel filter,.such'as.is described and claimed in my rst-mentioned -copending application. These parallellters do .not all need to be tuned to the same frequency, but may be tuned `anywhere in the frequency-range from about the third harmonic toxafrequency .of the order .of 900 cycles. An advantage. is obtained if the parallel-resonant frequencies lof' the three different parallel iilters are all different from each other, in that-phase-dirferences will thereby be produced in the various harmonic currentcomponents in the several supply-currents for the several loads, thus assisting materially in causing the objectionable interference-provoking harmonics to tend to cancel each other out, as pointed out in my second-mentioned copending application.

As shown in Fig. 1, therefore, the rectifierassembly which powers motor No. 1 is indicated, by wayof example, as having an untuned fourthharmonic parallel filter, consisting of a capacitor emacs? Clip and a damping resistor Rllp. When I designate the filter-frequency by a particular har- `monic order, I do not mean to limit myself to any particular precise tuning, as this is not necessary, and besides, the supply-circuit reactance varies under diiferent operating conditions and in accordance with the varying distance of any particular locomotive of the supply-source (not shown). For example, when I say the fourth harmonic, I means to include any frequency in the range between somewhere around the third and somewhere around the fifth harmonic. The parallel-resonant frequency in question is the frequency at which the lter-capacitor, such as G4p, is in parallel resonance with the power supply reactance as seen from the terminals of the filter.

By way of illustration, the second and third parallel filters, which are associated with the rectifier-assemblies for motors Nos. 2 and 3, in

Fig. l, are tuned for the eighth and twelfth harmonies, approximately, as indicated by the nuinerals and l2, in place of the numerals 4 which characterize the capacitor C and the damping resistance R of the respective parallel lters,

-these parallel-resonant filter-tunings being only -by way of example.

In accordance with one feature of my present invention, each of the parallel lters may be associated with one or more damped series filters,

`which are also connected across the pair of anode-leads 2I-22. In every such case, it will usually be desirable to have at least one series filter which is tuned approximately to the same frequency as the associated parallel filter, by which I do not mean that the frequencies must be precisely and exactly identical, but merely of the same order of magnitude, so that the impedance-humps or depressions of the respective ilter-impedances shall come somewhere around the same range of frequencies.

Thus, the fourth-harmonic parallel lter Clp-Rllp is shunted by a fourth-harmonic series filter which consists of a capacitor C45, an inductor Lllsand a resistor R45. The size of the series-lter capacitor C45 is determined solely by the size of the series-filter inductor Lis, these two reactors being tuned to be in series resonance with each other `at about the frequencies of (say) the fourth harmonic of the supply-circuit frequency. The series-filter damping-resistance, such as Rs, should preferably have a magnitude such as to give a breadth of tuning which approximately corresponds to the breadth of tuning of the associated parallel-resonant filter, so that the harmonic-shunting effect of the series filter, in the vicinity of the series-filter resonant-frequency, shall be spread out over a frequencyrange approximately corresponding to the frequency-range in which the associated parallel-.,-

resonant filter produces its most damaging harmonic-increasing effect whereby the parallel lter, if it were used alone, would tend to magnify the line-current harmonics in the vicinity of the parallel-lter resonance.

In like manner, the second parallel-resonant filter Cp-Rip in Fig. 1, `which is in parallel resonance with the supply-line inductance, up to the lter-terminals, at approximately the eighth harmonic of the supply-line currents, is shunted by a damped eighth-harmonic seriesvresonant filter C8S-Ls-R8s, which takes out the parallel-lter-augmented line-circuit harmonies in the vicinity of the parallel-resonant frequency which is somewhere around the eighth harmonic.

Since the effect of a parallel-resonant filter is to augment not only the line-frequencies in the immediate vicinity of the parallel-resonant frequency, such as the seventh and ninth harmonies, in the case of an eighth-harmonic parallel lter, but such a parallel filter also increases or augments all of the lower harmonics, such, for example, as the third and the fifth, it may be desirable, when the most perfect supplycurrent wave-forms are desired, to provide a second parallel-connected series-resonant filter which is tuned to some lower harmonic-frequency, 'that is, to some frequency between about the frequency of the third harmonic and some frequency which is below that of the frequency of the associated parallel-resonant lter. Thus, in the case of an eighth-harmonic parallel-lter, I may use, not only an eighth-harmonic series lter, but also a parallel-connected fourth-harmonic series lter C/Ss'-Lfis'-Ris, as shown in Fig. 1.

Thus, also, in the case of the third rectifierpowered motor-equipment of Fig. 1, the twelfthharmonic parallel filter Cip-Rlp is illustrated as being shunted lby three series-resonant filters, tuned approximately to the twelfth, eighth and fourth harmonics, respectively, as indicated at Gigs LZS Rl 2s', C85 L35 Rs" and CsH-LflsH-Rtsm so as to remove or reduce the third, fifth, seventh, ninth, eleventh, and thirteenth harmonics, while also reducing to a diminishing extent, some of the harmonics which are of a little higher order than the thirteenth. Of course, if there should be any even harmonics present, in the frequency-ranges of the series lters, they would be reduced teo.

The general effect of the parallel and series lters is indicated roughly in Fig. 2, only by way of example, and without any attempt at indicating optimum dampings or anything of the sort. For example, the eighth-harmonic parallel-resonant filter Cta- R39 will have an impedancefrequency curve of the general type which is indicated at Zp in Fig. 2. This practically shortcircuits out the high-order harmonics, but it has a damaging effect at the lower harmonic frequencies, particularly in the vicinity of the eighth harmonic. The general type of impedance-frequency curve which is obtained for a damped eighth-harmonic series-resonant lter, such as the filter CiS-LlS-RSS in Fig. 1, is indicated in Fig. 2 by the curve ZS, which shows that it shorts out harmonics in the frequencies which are close to the range in which the series-resonant tuning is provided, while having relatively small effect on the frequencies which are remote from the series-resonant tuning.

The operaticn of my system, as shown in Fig. 1, can therefore be briefly summarized as follows. The supply-line reactance during the angle of overlap (which is an inductive reactance), up to the terminals of the rectiers 2l, 22determines the angle of overlap of the two rectifiers of each pair of rectiilers. Preferably this reactance should be sufcient to make the angle overlap at least about 20 during each half-cycle during maximum load-conditions, although in some cases, an angle of overlap as high as 40, or more, might be desirable, while, at other times, any material amount of angle of overlap might be acceptable, when other features are used in accordance with my present invention. The overlap-controlling line-reactance always includes the anode reactors 2l and 22", because these reactors are always connectedin series with the supply-side of the rectiners 2| and 22.

In the case of the parallel-filters, such as CA1P-R513 Cgil-*Rain and CZp-REp, the linereactance with which the parallel-.filter capacitors will be in parallel resonance is the linereactance as seen from the terminals of the parallel lter. This filter-effective line-reactance will, or will not, include the anode-reactors 2i and 22, or either of them, depending upon whether saidline-reactors are included on the line side, cr the rectier side, of the parallellter terminals. The line-reactance has no effect on the tuning or adjustment of thefseries lters.

The mutual couplingbetween the anode-reactor windings 2i and 22, and their series-circuit connection with respect to the circulating or commutating currents which ilow when both rectiers are conducting, are desirable and novel features. Because of the presence of air in the magnetic circuit, the two` reactor-windings, for the best results, have to have their individual coils carefully interleaved, as distinguished from an arrangement whereby each ofthe two windings is a compact unit spaced from the other winding. The mutualcoupling makes'the coremagnetization go from a maximum positive flux to a maximum negative flux in successive halicycles, instead of going from positive to zero, or from negative to zero, as would be the case if two separate anode-reactors were used, without any mutual coupling between them. The. size of the double reactor is thus reduced, as distinguished from what would have been required if two uncoupled reactors were used.

The high series-circuit, or commutatingcurrent, reactance is desirable, in many rectifierpowered vehicles, in order to produce some desired slope or regulation in the speed-tractive effort curves of the locomotive or car, and also to produce a large angle of overlap, which is necessary in order to reduce inductive telephoneinterference, as pointed out in my first-mentioned copending application. But of even greater importance is the fact that a large series-connection circulating-current reactance isneeded for the purpose of limiting the magnitude of backfirecurrents, in case one of the tubes or rectiiiers should become conducting in the wrong direction, after its current-zero state has passed. It is very necessary to limit such backiire-currents, in order to avoid damage to the faulty rectifier, and also to avoid a power-outage condition due to the operation of overcurrent fuses or breakers (not shown).

The introduction of a considerable portion of the supply-circuit reactance on the secondary side of the power-transformer Il, by means of the ancde-reactance 2l and22", is particularly useful, also, in a system in which a plurality of similar separate rectier-powered motorequipments are operatedirom the same powertransformer. If there are N rectier-equipments, energizing N direct-current meters from the same power-transformer Il, then the anode-reactors 2l-22 are N times as effective as a backfireprotective means, when they are connected in series with the individual pairs of anode-leads 2|', 2.2', on the secondary side of the powertransformer Il, than would be the case if an equivalent amount of reactance (for some predetermined v-alue of regulation in the locomotive speed-tractive eiort curves), had been built into thepower-transformer l I, or had been connected in series with the primary circuit of said transformer. This is true, because, when a backring condition arises, it occurs in only one rectifier at a time, as a rule, and hence an individual backfire-suppressing reactance-coil, which is specially included in the circuit of that one transformer, is much more effective in reducing the backfire-current than if the reactor had been placed in the primary circuit where it is traversed by the currents drawn from all N rectierassemblies where, for a predetermined regulating-effect and angle of overlap, it could have only times the reactance it can have in lthe single motor-current circuit.

The use of individual anode-reactors, on the secondary side of the power-transformer is also desirable, in my preferred combination in which a plurality of rectier-powered-motor-circuits are energized from a common power-transformer, because such anode reactors make possible the choice of different amounts of supply-circuit reactances. which are eiectively connected in series with different ones of the plurality of rectifier-assemblies. This makes it possible to cause the several rectifier-assemblies to draw supply-currents which are slightly out of phase With each other, thus throwing the high-order harmonics considerably out of phase with each other, and making it possible to make critical groups of harmonics substantially balance each other out, so that they do not appear in the supply-circuit 'I where they would cause inductive interference in the adjacent communication system 8.

To a similar end, the connection of the parallel lters Chl-R41), CSp--Rp and Cl2p-Rl2p Yon the rectifier or load-side of the anode-reactors 2I"-22 is quite desirable, in a system wherein a plurality of separate rectifier-powered motorcircuits are energized from the same powertransformer Il. This connection makes it possible to use different sizes of parallel-lter capacitors G4p, Cp and Ci2p, which not only tune the various parallel rectiers to different frequencies, thus in itself tending to throw the harmonies out of phase with each other, but the very considerable line-frequency capacity-currents, which are drawn by these parallel-lter capacitors of different sizes, cause the supplycurrents which are drawn by the several rectierassemblies to be out of phase with each other, having different power factors, and this phasedisplacement, when multiplied by the order of any particular harmonic, will produce a rather signicant harmonic phase-displacement, when harmonics of the higher order are considered, as pointed out in my second-mentioned copending application.

It is also desirable, as set forth in my rstmentioned copending application, to make the total inductance of each of the direct-current circuits l3-I4-l5-l'l suiciently small to permit at least a 20% ripple to be present in some part of said direct-current load-circuit during maximum short-time load-conditions, dening the percent ripple as one-half of the difference between the maximum and minimum instantaneous values of the direct-current wave, divided by one-half of the sum of said maximum and minimum values. In some cases, at least a 40% ripple may be desirable. These are unusually high ripple-contents for the direct-current loadcircuit of any rectifier-assembly.

This high ripple-contact is made possible, in rectifier-powered locomotive-installations and the like, because the direct-current load-circuits of srch' installations are not subject to inductiveinterference considerations, because such circuits do not extend in inductive-interference relation to any communication circuit. Furthermore, it has been found that the direct-current motors l, 2 and 3 are much more tolerant of ripples than was once assumed to be the case. Thus the ripples do not substantially hurt the motors, and they are very useful in reducing the harmonics which are reilected back into, or produced in, the alternating-current supply-circuit 1, which is very much subject to inductive-interference considerations.

Fig. 3 is presented for the purpose of illustrating the fact that practically all rectifier-powered trolley-energized vehicles will use alternatingcurrent or supply-circuit voltage-control, for controlling the rectified voltage which is applied to the motors, and thus controlling the speed of the motors. Practically always, therefore,the powersupply transformer, such as the transformer which was shown in Fig.i l, will be a variablevoltage transformer, and practically always, the variable voltage will be obtained by means of secondary taps 23 and 24 such as are shown on the power-transformer 4in Fig. 3. Herein lies another advantage of using my anode-reactors 2| and 22", connected inseries-cireuit relation in the anode leads 2| and 22 on the secondary side of the power-transformerl i. This 'is' particularly true on low-voltage tap-connections, when the effect of the transformation-ratio is to make the line reactance, as seen from the secondary-winding taps, very small indeed-being relatively negligibly small, whereas `the effective reactance of the anode-reactance 2|-and 22" remains substantially the same under these lowvoltage conditions as under the high-voltage conditions which prevail under normal running-conditions of the motors. l

Hence, my use of the anode-reactors 2|-22 provides a system where there is some'substantial amount of effective supply-circuit reactance in series with the input-sides of the rectiers 2| and 22, even during the lowest-voltage operatingconditions of these rectiners. This supply-circuit reactance is desirable, in order to provide a certain amount of voltage-regulation, by which is meant that the voltage which is applied to the rectifier-powered motors willv drop or fall ofi by a certain amount as the motor-load increases, and conversely, the motor-voltage will gradually rise, by itself, as the motor-speed increases during the acceleration of the vehicle, this effect being obtained without changing eitherof the secondary taps 23 or 24 of the power-'transformer l. A certain amount of this voltage-regulation eiect is quite desirable in reducing the number of transformer-taps, and the number of controller-notches which are'required on the motor-controller (not shown) which controls these taps.

Fig. 4 is a simplied diagrammatic view which is illustrative of the possibility of connecting the various lters (parallel lters and/or series lters), in different positions with respect to the input and output sides of the several anode-reactances 2|-22 of the various rectifier-equipments which are energized from any given powertransformer I on a locomotive or other rectierpowered trolley-energized. vehicle, In Fig. 4, the

. 12 Y number of direct-current traction-motors is shown as six, which is a more usual number than the three motors which are illustrated in Fig. l, these six motors being indicated, in accordance with the usual convention, by the numbers to 6, respectively.

In Fig. 4, it will be observed that two parallelfilter circuits 3| and 32 are shown on lthe powerinput sides of the several anode-reactors 2i, 22"; that is, connected across the secondary winding l2 of the power-transformer These two parallel filters may be tuned either to apprcximately the same frequency as each other or to dierent frequencies, within the frequency-range between the third harmonic and 900 cycles.

In Fig. 4, also across the input-sides of the several anode-reactors 2|-22", that is, across the power-transformer secondary-winding i2, I have shown a series-lter circuit 33, which might be tuned to approximately the same frequency as one or both of the parallel lters 3| or 32, or at any lower harmonic frequency (higher than the supply-line frequency).

Fig. 4 also shows, by way of examples, other places where both the parallel and series lters could be connected, either instead of the ltercircuits 3|, 32 and 33 which have just been described, or in addition thereto. Thus, the rst pair of single-phase power-supply leads or anodeleads 2|', 22', of the No. 1 motor, are shunted, on the rectifier-side of the anode-reactors 2|"-22, with a parallel-filter circuit 33 and a series-iilter circuit 35. A similar series-filter circuit 35 is shown also, in a similar connection, across the anode-leads of all of the other rectifier-assemblies, which energize motors Nos. 2 to 6, respectively. A' .parallel-filter circuit 3s, connected as justdescribed, is shown also for the rectifier-assemblies of the motors Nos. 3 and 4. The rectifier assembly for the No. 6 motor is shown as havinga parallel-filter circuit 36 which is connected with one filter-terminal 3B on the power supply .side of one of the anode-reactor windings, such as 22,', while the other filter-terminal 36" is connected on the rectifier side of the other anode-reactor winding 2|. In the case of the rectier-assemblies for energizing the motors Nos. 4 and 5, Ihave shown two crisscrossconnected parallel-filter circuits 36 and 3l, each having its filter-circuits connected on opposite sides of the respective anode-reactor windings.

These various filters 3| to 31 may be tuned in parallel and series resonance, as the case may be, at any of the frequencies which have already been discussed, either at all the same frequency, or at diierent frequencies, for the purposes already stated.`

I have found that there is a considerable difference in theoperation of the lters, according to their location with respect to the anode-reactors 2|22.l This is particularly true of the parallel lters, in which the'lter-capacitors draw and supply material amounts of filter-currents during the respective capacitor-charging and capacitor-discharging periods of each half-cycle of the line-frequency currents.

These filter-connection eiects are closely related to the effects of the anode-reactances and the effect of using a loW-reactance power-transformer Il, as will now be described. In fact, it is desirable thatthe power-transformer reactance shall be negligibly small, as compared with the reactances of the various anode-reactors 2 I" and .22", so that, during the coinmutating period,

when circulating Current is owing through both of the rectiers'zl and 22 ofrany vgiven rectierassembly, the impedance-drop, or the voltage necessary to drive the commutating current through the reactance of the commutating circuit, will thus he consunied substantially entirely in the. anode-reactors 21"-22", and substantially none of this commutating-voltage drop will he obtained across 'the terminals oi the power-transformer secondary-winding i2. Hence, the largeness of the angle oi overlap will-cause very little voltage-regulation in the transformer windings. This feature is desirable on account of other loads on the transformer,

Thus, when the terminal-potential of the righthand half of the secondary windingv l2 nrsthegins to be more positive than the terminalpotential of the left-hand half, a circulating currentwill flow throughthe anode-reactor winding 22" of the anode-lead 22', the newly fired rectifier 22, the common cathodeucircuit itil, the previously firing rectifier 2l, and the other anode-reactor winding 2i" oi the anode-circuit ill', and thence back to the'power-transformer secondary-winding i2.

The eiects just described arev enhanced by the presence ofthe filters, particularly any parallel filters such as 3l, 32, 35i, 35 and 3i?, because the capacitors of theseV parallel nlters, as a rule, draw larger charging and discharging currents than do the capacitors of the series filters 33 and 35 which have material inductances associated in series resonance therewith, as parts of their filtercircuits.

T us, when onlyy one filter is used, such as the parallel-resonant nlter-circuit 3 iy (Fig. e) located on the power side of the anode-reactors E l 22", the alternating-current waveeform, as deternined in the primary circuit of the power-transformer il, has a shape somewhat oi the type shown at V31 in Fig. 5, which does'not necessarily represent the optimum design-conditions. However, ir" the nlter had been connected on the rectiiier-side of the anode-reactors 2i-22, as shown at 3d in e, the primary-circuit waveform would have had an appearance suchas is depicted at Vn in Fig. 5. It will be noted that the humps and hollows inthe two wave-forins-Vsi and V34 are approximately le)o out or phase with each other, so that, if one of the rectiner'assemh-lies draws a primary current of onetype, and another rectifier-assembly draws primary current of the other type, or if several recti'erassemblies draw currents of various intervening types, there will oe a considerable amount of wayeesrnoothing effect, due to the simultaneous flowing of the primary currents of all of the several types. This is true if the different motors are energized from different step-down transformers il, or even from different secondaries (not shown) on the same'transformer, or if the mutual transformer-reactance in the chargingcircuits of the diierent nlter-capacitors is quite small as compared to the anode-lead reactances. ln other words, if thereactance of the trans- 'former l i is neeligihly small as compared to the anode lead reactances, or with respect to the power-line reactance, the above-described rip plecanceling enects can he expected in the waveform of the total line-current.

It will he noted that' the humps antidepressions on the wave-orins V31 and V34c in Fig. 5 are of rather low harn1onicorders (considerably under the thirteenth harmonic, for exampie) ,so that any harmonic-depressing effect, due `to the varied ccnnectionsof the lters with respect to the-'power and load sides of the anode-reactors, would be of benent mainly from the standpoint of obtaining more nearly sinusoidal total currents, than from the standpoint of suppressing the high-order harmonics which are the ones which cause the most telephone-interference troubles. Both the higl1-order and the low-order harmonics ought to be suppressed, however, and my present connections, as shown in -Fig. 4, are of considerable benefit in this regard.

The positions of the series-lter circuits, such as thecircuits 33 and of Fig. 4, with respect to the power andload sides of the anode-reactors 2l-22, are also signicant. Harmonics are generated in the inductance of the direct-current motor-circuit and enter the transformer-secondary l2 at its midtap connection. When only one series-filter is used, and placed on the rectiiiernside, as shown at in e, it provides a path for the circulation of ha. ,nonies flowing in a closed path-including the ltransiorrnersecondary i2 and the anode-reactors 2 l and 22, sorfar as its particular tuned frequency is concerned, or any harmonics close to that frequency. The mutual reactance between the anode-reactors 2l and 22 assists the mutual reactance between the two halves of the transformer-secn ondary l2 in forcing these low-order harmonic components of the circulating currents to iiow in equal strengths and opposite directions through the two halves or the transormensecondary l2. The capacitor oi the lter 35 or 3Q inust charge up at the oeginnine' of each line-frequency halfcycle, through a charging circuit including the anode-reactance, the transformer-reactance, and the line-reactance, and the sum of these chargingecircuit reactances L determines the critical damping-resistance RzL/C of the series filter 35. The filter-capacitor discharges through a discharge-circuit which includes the almost-Zero reactance of the rectifier-tubes 2l, and

On the other hand, if only one series nlter is used, connected on the power-input side of the anode-reactor transformer 2l-22", as shown at33 in Fig. e, the mutual anode-reactor coupling does not assist the flow of the harmonics in the two halves of the transformer-secondary l2. On the contrary, the voltage-drop due to the flow' of the harmonic currents through one of the anodereactors 2i or Eil", as the case may he, adds to the volts per turn which must be generated in the active half of the transformer-secondary l2, in order to force the harmonics to low through the inactive half and through the series filter 33, and thence through the aforesaid anodereaotor 2 or 22, and the direct-current circuit, and haci; to the midpoint of the transformer-secondary l2. Thus, the anode reactor exercises a voltage-drop effect, in the case of the power-sideseries filter hinstead or having a harmonic-balancing effect as in the case of the rectifier-side series nlter 35.

The capacitor oi the power-side series lter 33 charges through only the line-reactance and the transformerreactance, and hence it has a smaller critical dampingnresistance (and hence a smaller' resistance-loss for a given size of capacitor), than is the case with. the rectifier-side series lter 33. The power-side ilter S5 discharges its capacitor through the anode-reactor reactance, as well as the tuhe-reactance. If the reactances of the charging and discharging circuits of this power-side filter 35 are made equal, the critical damping resistance will be the same for charge and discharge, and will be less than the damping 15 resistance which is required in the rectier-side lter 33.

The same remarks regarding the charging and discharging circuits, and the necessary amounts of damping resistances, apply to the power-side and rectifier-side parallel-filter circuits, such as the circuits 3l, 32 and 34 of Fig. 4, as well as to the series-lter circuits 33 and 35 which have just been discussed.

One of the disadvantages of the parallel-resonant iilters of my first-mentioned copending application was the necessity for adding a sufficient damping-resistance, such as the resistance Rlip of Fig. 1, to suitably limit the charging and discharging currents of the lter-capacitor, such as the capacitor Clip. These parallel-filter damping-resistances entail a certain amount of continuous energy-loss, because the parallel-filter capacitors (such as Clip) draw really material amounts of line-frequency capacity-current from the line. According to my present invention, it is believed that smaller and less critical amounts of damping-resistances can be used, in the parallel filters, if some of these lters are placed on the power side, and some on the rectier side,

or crisscrossed in various ways, with respect tov the anode-reactance, as shown and discussed in connection with Fig. 4.

An important advantage of rectier-powered locomotives is that they are operable on 60-cycle lsupply-circuits, so that they do not need the special 25-cycle power which is required by alterhating-current rectications using series commutator alternating-current motors. When 60- cycle power is used, my various filters and lterconnections become all the more important, because of the greater magnitude of the harmonic interference-diiculties which arise when a 60- cycle supply-circuit is used, rather than a 25- cycle supply-circuit.

Many railway-electrications already in existence, however, use a 25-cycle power-supply, and it is necessary for my invention to be usable successfully on such existing systems. In many such 25-cycle railway-electrifications, cab-signals are used (not shown), which are usually operable on 100 cycles, corresponding to the fourth harmonic of the supply-frequency. In such cases, my fourth-harmonic series-resonant filters, such 3S the filters CArLQS-RAS, CIQS-LQSRS, and C4s-Ll3s-R4s", will usually be necessary or at least quite desirable, in order to avoid interference with the cab-signalling system, when using rectifier-powered locomotives on the electrified railway-system.

The relative advantages of the positions of the filter connections, whether on the transformerside of the anode-reactors 2i-22" or on the rectiiier-side, depend upon a number of considerations. In general, when the rectifiers 2| and 22 have substantially no ring-delays, a rectierside filter, such as the lter 34 of Fig. 4, is slightly more efficient as an interference-noise-suppresser, than a transformer-side iilter such as 3l or 32. However, if any material amount of delayed firing is used in the rectiers 2l and 22, the increased amount of inductive interference which is caused by such delayed ring is much better suppressed 'by a transformer-side iilter, such as 3l or 32, than by a rectier-side lter such as 3d.

While I have described only a few extremely simplified and illustrative or exempliiying forms of embodiment of my invention, designed to beindicative rather of the general principles of my invention than showing precise details of the actual complicated connections such as would be used in practice, I wish it to be understood that my invention is not limited to the illustrated forms or circuits. I desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language.

I claim as my invention:

1. A rectifier-powered equipment, comprising, in combination, a direct-current load-means, a rectifier-assembly comprising at least two singlephase rectifying devices of a type which becomes substantially non-conducting, after a conducting period, only in response to a currentdecrease to substantially zero; a pair of singlephase power-supply leads, connected to said two rectifying devices, respectively, at rectiiyingdevice terminals of one polarity; circuit-means for connecting the other terminals of said two rectiiying devices to one terminal of said directcurrent load-means; the supply-circuit inductance, up to said iirst-mentioned terminals of said two rectifying devices, being suilicient to cause the conducting periods of said two rectifying devices to overlap for at least about 20 during each half-cycle during maximum shorttime load-conditions; an untuned parallel-resonant lter, connected in parallel-circuit relation across said power-supply leads, with at least a substantial part of said supply-circuit inductance on the power-supply side of said parallel-ter connection, said parallel-resonant iilter comprising a capacitor and a damping resistance, said capacitor having such capacitance as to be in parallel resonance with the supply-circuit inductance as seen from the terminals of said parallel-resonant filter, at a frequency in the range between the third harmonic of the supply-circuit and 900 cycles, during at least some operating-conditions; and a damped, series-resonance, tuned filter, connected in parallel-circuit relation across said power-supply leads, with at least a substantial part of said supply-circuit inductance on the power-supply side of said series-filter connection, said series-resonance filter comprising a capacitor and an inductor which are tuned to approximately the same frequency as said parallel-resonant'lter 2. The invention as defined in claim l, characterized by the parallel-lter resonance being in the frequency-range between the third and iifth harmonics of the supply-circuit.

3. The invention as defined in claim l, characterized by the parallel-lter resonance being in the frequency-range between the fifth harmonic of the supply-circuit and 900 cycles; in combination with a lower-frequency damped, seriesresonance, tuned filter, connected in parallelcircuit relation across said power-supply leads, with at least a substantial part of said supplycircuit inductance on the power-supply side of said series-filter connection, said lower-frequency series-resonance filter comprising a capacitor and an inductor which are tuned to a frequency in the range between the third harmonic of the supply-circuit and the frequency of the parallel-resonant lter.

4. The invention as dened in claim l, characterized by each of said two rectifying devices becoming conductive when the supply-circuit voltage reaches a relatively small value in the polarity which is conductive for that rectifying device.

5. The invention as dened in claim 1, characterized by the direct-current load-circuit having a total inductance which is suniciently small to permit at least a 20% ripple to be present in some part of said direct-current load-circuit during maximum short-time load-conditions.

6. The invention as defined in claim l, characterized by said power-supply leads including a reactor-transformer having two closely coupled, substantially identical-turn windings and a common magnetic circuit including an air gap, said two reactor-windings being serially included in said power-supply leads in such polarity that the two reactor-windings are in series, producing a maximum reactance in the reactor-transformer, for the commutating currents which flow between said two rectifying devices during their overlapping conducting periods.

'7. A rectifier-powered equipment, comprising, in combination, a direct-current load-means, a rectifier-assembly comprising at least two singlephase rectifying devices lof a type which becomes substantially non-conducting, after a conducting period, only in response to a currentdecrease to substantially zero; a pair of singlephase power-supply leads, connected to said two rectifying devices, respectively, at rectifyingdevice terminals of one polarity; and circuit means for connecting the other terminals of said two rectifying devices to one terminal of said direct-current load-means; the supply-circuit inductance, up to said first-mentioned terminals of said two rectifying devices, being suiiicient to cause the conducting periods of said two rectifying devices to overlap during each half-cycle during some load-conditions; and said powersupply leads including a reactor-transformer having two closely coupled, substantially identical-turn windings and a common magnetic circuit including an air gap, said two reactorwindings being serially included in said powersupply leads in such polarity that the two reactor-windings are in series, producing a maximum reactance in the reactor-transformer, for the commutating currents which iiow between said two rectifying devices during their overlapping conductng periods.

8. The invention as defined in claim 7, characterized by said overlapping conducting periods lasting for at least 20 during maximum shorttime load-conditions; and further characterized by the direct-current load-circuit having a total inductance which is suiiiciently small to permit at least a 20% ripple to be present in son e part of said direct-current load-circuit during maximum short-time load-conditions.

9. The invention as defined in claim 7, in combination with an untuned parallel-resonant filter, connected in parallel-circuit relation across said power-supply leads, with at least a substantial part of said supply-circuit inductance on the power-supply side of said parallel-filter connection, said parallel-resonant lter comprising a capacitor and a damping resistance, said capacitor having such capacitance as to be in parallel resonance with the supply-circuit inductance as seen from the terminals of said parallel-resonant filter, at a frequency in the range between the third harmonic of the supply-circuit and 900 cycles. during at least some operating-conditions.

l0. The invention as defined in claim 9, characterized by said parallel-resonant filter being connected on the power-supply side of said reactor-windings.

ll. The invention as defined in claim 9, characterized by said parallel-resonant filter being con ected between the power-supply side of one Cil 18 of said two reactor-windings and the rectiiier side of the otherI reactor-winding.

l2. The invention as defined in claim 9, characterized by having two crisscross-connected parallel-resonant filters as defined in claim 9, each connected between the power-supply side of one oi' said two reactor-windings and the rectifier side of the other reactor-winding.

13. The invention as defined in claim 9, characterized by said parallel-resonant filter being connected on the rectifier side of said reactorwindings.

14. The invention as defined in claim 9, characterized by having two parallel-resonant lters as dened in claim 9, one on the power-supply side, and the other on the rectifier side, of said reactor-windings.

15. The invention as denned in claim 9, in combination with a damped, series-resonance, tuned filter, connected in parallel-circuit relation across said power-supply leads, on the powersupply side of said reactor-windings, said seriesresonance filter comprising a capacitor and an inductor which are tuned to approximately the same frequency as said parallel-resonant filter.

16. The invention as defined in claim 9, in combination with a damped, series-resonance, tuned iilter, connected in parallel-circuit relation across said power-supply leads, on the rectifier side of said reactor-windings, said seriesresonance filter comprising a capacitor and an inductor which are tuned to approximately the same frequency as said parallel-resonant lter.

17. Rectifier-powered equipment, comprising, in combination, a plurality of similar direct-current load-means, a rectiiier-assembly for each of said direct-current load-means, each rectierassembly comprising at least two single-phase rectifying devices of a type which becomes substantially non-conducting, after a conducting period, only in response to a current-decrease to substantially Zero; a pair of single-phase power-supply leads for each of said rectifier-assemblies, each pair of power-supply leads being connected to said two rectifying devices, respectively, of their own rectifier-assembly, at rectifyingdevice terminals of one polarity; circuit-means for connecting the other terminals of said two rectifying devices to one terminal of their associated direct-current load-means; a common single-phase supply-circuit; and energizingmeans for energizing said plurality or" pairs of power-supply leads from said common singlephase supply-circuit; each pair of power-supply leads including a reactor-transformer having two closely coupled, substantially identical-turn windings and a common magnetic circuit including an air gap, said two reactor-windings being serially including in said power supply leads in such polarity that the two reactor-windings are in series, producing a maximum reactance in the reactor-transformer, for the commutating currents which flow between said two rectifying devices during their overlapping conducting periods.

18. The invention as defined in claim 1'7, characterized by said energizing-means being a common transformer for energizing a plurality of rectier-assemblies.

19. The invention as defined in claim 1'?, characterized by a plurality of said reactor-transformers having materially diverse reactances.

20. The invention as defined in claim 17, char.- acterized by a plurality of untuned parallelresonant nlters for each of a plurality of pairs ageeaeee of power-supply leads, each parallel-resonant filter being connected in parallel-circuit relation across their power-supply leads, each parallelresonant lter comprising acapacitor and a damping resistance, said capacite-r having` such capacitance as to be in parallel resonance' with the. supply-circuit inductance as seen from the terminals of said parallel-resonant iilter, at a frequency in the range between the third harmonic or the supply-circuit and 900 cycles, dur-r ing at least some operating-conditions.

21. The invention as defined in claim 2), characterized by a plurality of parallel-resonant filters being diversely connected with respect to the reactor-windings of their respective power-supply leads.

22. The invention as dened in claim 20,. characterized by a pluralityv of parallel-resonant filters being diversely resonant in parallel resonance at dinerent frequencies.`

23. The invention as defined in claim 22, characterized by each of a plurality of said` diversely resonant parallel-resonant filters having atleast one' of its terminals connected toits associated power-supply lead on the rectifier side of the associated reactor-Winding.

24. The. invention as delinea. in claim 23 in combination with a plurality ci. dampedseriesresonance, tuned filters, connected inv parallelcircuit relation across a plurality of pairs of power-supply leads in complementary relation to' said plurality of diversely resonant parallelresonant filters, each series-'resonance lter comprising a capacitor and an inductor which are tuned. to approximately the saine frequency as their associated parallel-resonant filter.

25. A rectifier-powered equipment, comprising,

2@ in combiriatien;A :indirect-current lead-means, a rectier-as'sembly comprising atleast two singlephase rectifying devices ora type' which. becomes substantially non.-conducislng, after aconclucting period, only iny response to a current-decrease to substantially zero; al pair ci Single-phase perversupply leads', connected te said two. ifc'ctiflying devices, respectively, at rectifying-device terminalg of one polarity; circuit means for connesting the otner'terminals or said two rectifying devices tc one terminal of said direct-current load-means; a single-phase supply-circuit; ane a Variable-Voltage transformer connected between said single-phase' supply-circuit and said single-phase power-supply' ieads; said powersupply leads including a reactor-transformer having" tivo closely coupled, substantially identicalturn windings' and a commenmagnetic circuit including an air gap', saidv two reactcr-xvindingsbeing serially included in said power-supply leads inl such polarity that the tivo reactor-windings arev in' series, `producing a maximum ieactance in the reactor-transformer, for the ccrnmutatine' currents which new between said two rectiyingi devices during their overlapping conducting periods.

LLGYJ. BAPLB;

References Cited tliA le 0f this patent UNITED STTES QIYEITSv 

